Rigging two-handed weapon in Blender 2.8
Create a root bone for your gun, create 2 children bones, one for handle 1, one for handle 2:

In Pose mode, give each of your hand controller a Child Of constraint, with the handle 1 and handle 2 as Target, click on Set Inverse:

Move each controller close to the handle, move the fingers so that it looks like they are grabbing the handles, keyframe:

Move your gun, the hands should follow: